Putzmeister involved in the largest infrastructure project in Scotland

Construction of a new road bridge over the Firth of Forth is currently Scotland's largest infrastructure project, and is intended to largely replace the current Forth Road Bridge. In July 2013, after a lengthy preparation period, the concreting work got underway on the foundations of the bridge’s three towers. Across all three towers, a total of 28,669m3 was poured forming giant plugs on which to base each tower’s foundation. On the South Tower within a period of 15 days, 16,869 m³ of concrete was poured – the longest continuous concrete placement under water to date. 7,400m3 and 4,400m3 of concrete were poured on the North and Centre Towers respectively. This was a task in which not only Putzmeister equipment, but also the expertise of our engineers was needed.

Edinburgh. In December 2010, the Scottish Parliament decided to have a new bridge built over the Firth of Forth. This sea loch Situated on the east coast of Scotland between Edinburgh and Fife, the estuary of the River Forth flows into the North Sea. It separates the Scottish Lowlands from the Highlands. Due to corrosion damage to the suspension cables, it is likely that the heavily-used, dual carriageway Forth Road Bridge (opened in 1964) would have had to be partially closed to traffic from 2017. Directly beside it, the new Queensferry Crossing bridge will carry the majority of the daily road traffic in future. A stable steel cable design with a total length of 2.7 km will enable uninterrupted traffic flow, even in high winds thanks to modern wind shielding installations. The project client is Transport Scotland, the Scottish Government’s transport agency. The client awarded the construction contract to a Joint Venture called Forth Crossing Bridge Constructors (FCBC). The new bridge is scheduled to be open to traffic by the end of 2016. The value of the project order is around 950 million (£790 million) Euro for FCBC.

The concrete is pumped from the barge using a stationary concrete pump through the first placing boom into the scond boom.

Preliminary work for construction of the bridge towers Before concreting the foundations, there were comprehensive preparations to be made. As early as summer 2012, work started on towing three caissons (up to 1200 t) to the planned locations of the bridge towers, in order to sink them approximately 40 m deep into the waters of the Forth, until they sank into the seabed. Not an easy task at a maximum total height of 30 m and a diameter of 32 m. With a top section which is meant to serve as a temporary bulkhead until the lower bridge tower segments are finished, these protrude approximately 4 m out of the water.

Starting the longest continuous concrete placement under waterAugust 2013 saw the start of work on the longest continuous under water concrete placement and the third-largest concreting project worldwide. The concrete for the bridge tower foundations was brought on-site from one of the most modern mixing systems (operated by FCBC) in Britain situated in Rosyth Docks near the new bridge. The concrete operation at the South Tower, the last of the three tower foundations to be completed, was the largest underwater concrete pour in the world.

Four barges, each containing six mixer drums (each of which had a capacity of 12 m³), transported the material first to the location of the southern bridge tower. The barges chalked up a total of 273 journeys and a total distance of 1800 km. From a land-based site, Putzmeister truck-mounted concrete pumps had transported the concrete to the mixer drums. Once arrived at their destination, a stationary BSA 2110 HP D concrete pump fastened in the middle of each barge pumped the material into another truck-mounted concrete pump that was also transported. This was then used to concrete the foundation. For concreting the pylons, a BSA 2110 HP D stationed at the bridge tower foundation is charged from the truck-mounted concrete pump. The BSA 2110 HP D in turn pumps the concrete via a pipeline to an MX 32-4 type stationary boom mounted on the bridge tower itself.

It follows that the success of the concreting decisively depended on the problem-free operation of the six Putzmeister truck-mounted concrete pumps in series M 36-4, M 42-5 and M 47-5. The machines reliably met all expectations: With an average delivery rate of 47 m³/h, they placed up to 720 m³ of concrete per day.

For the production of the 25 m thick foundations of the southern bridge tower, this method was used to pump a total of 16,869 m³ of concrete non-stop over 15 days. The concrete was set to free-flowing at a water-cement value of 0.36, so that it did not need to be compacted. The same method was used to construct the foundation of the northern bridge tower. Not so with the centre tower, which will be erected on a rock halfway across the river.

Simultaneous construction of the actual towers is now underway in 4 m sections, using self-climbing formwork, and small distribution arms fitted in the formwork (a type of two-arm crane with attached hose). The BSA 2110 HP D, stationed at the bridge towers, supplies the distribution arms with concrete. By the end, 72,000m³ of concrete will have been poured for the foundations of the three bridge towers alone, and by the time the two-way bridge is finished, a total of approximately 150,000 m³ of concrete will have been poured.

Pump tests determine the optimum pumpability of the concreteFor the upcoming concreting of the bridge towers, Putzmeister project engineers organized and were present at a pump test at the start of 2014. The aim was to determine the pumpability of different concretes and the design of suitable concrete pumps and pipelines on site. 165 m of pipeline was erected for this purpose, in order to simulate the maximum height of the highest pylons (210 m). A BSA 2110 HP D pumped the different types of concrete through the pipeline and then through a truck-mounted concrete pump. In this way, the engineers were able to simulate different delivery rates (10, 20 and 30 m³/h), and the effects on the pressure requirements of the concrete. In parallel, the Sliding Pipe Rheometer, or “Sliper” for short (developed by Putzmeister), measures the coefficients of friction of the concrete.

Forth Road Bridge – the most important link between the Lowlands and HighlandsThe Forth Road Bridge is a dual carriageway bridge with a steel design. This suspension bridge was erected between 1958 and 1964, and was the largest bridge of its kind in Europe at the time. It is 2.5 kilometres long, and alongside the famous Forth Rail Bridge, spans the Forth Estuary between North Queensferry and South Queensferry. It represents the most important link between the Scottish Lowlands and Highlands. A total of almost 47,000 t steel was put in place, including a total of almost 50,000 kilometres of wire cable for the supporting structure. In 1964, HM the Queen presided at the bridge opening ceremony. Since 2001, the Forth Road Bridge has been a listed structure. From 2017, it will be closed to the majority of traffic, but will remain open to buses, taxis and cyclists as a public transport corridor.

The right pipeline range from Putzmeister for every applicationPredicting the expected concrete pressures and wear behaviour of concrete using pump tests and Sliper tests enables economic composition of the pipelines, among other things. The aim is to only plan for high-pressure resistant pipes in locations where they are absolutely necessary. This minimizes costs for pipelines. At the same time, they should be designed so that they need not be replaced due to wear during the construction phase.

Putzmeister supplies pipelines for many different uses and applications. On the one hand, they differ in terms of their connection system, and on the other in their wall thickness and therefore in their attainable concrete pressure level. On truck-mounted concrete pumps, the pipes must be moveable and the couplings easy to open, for example. The SK coupling system, with its maximum pressure level of 85 bars, is recommended here.

For vertical and horizontal pumping, the pipelines are generally fitted with the coupling system ZX. ZX pipes are available in pressure levels 85 bar, 130 bars and 200 bars. With wall thicknesses of up to 11 mm, these pipes provide a very good wear reserve. In addition, the ZX system guarantees the absolute leak tightness of the system, and minimizes wear at the join, as well as the risk of blockages.

What is a caisson?A caisson (also called a drop shaft) has a hollow, cylindrical or square design, and serves as a foundation or working space under water. It is therefore called a drop shaft, because it is usually constructed on land and then “dropped” under water for certain types of work and concreting under water. The hollow structure is lowered to the bed of a body of water under gravity, and the ground material contained within the space is removed by surface, barge-mounted excavators. In this way, the caisson can be brought ever lower, until it reaches the required depth. The cavity (e.g. in the case of bridge towers) is filled with concrete.